U.S. patent application number 11/991072 was filed with the patent office on 2009-06-18 for method for electrolyzing molten salt, electrolytic cell, and process for producing ti using said method.
Invention is credited to Katsunori Dakeshita, Masahiko Hori, Tadashi Ogasawara, Kazuo Takemura, Toru Uenishi, Makoto Yamaguchi.
Application Number | 20090152122 11/991072 |
Document ID | / |
Family ID | 37808663 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090152122 |
Kind Code |
A1 |
Ogasawara; Tadashi ; et
al. |
June 18, 2009 |
Method for electrolyzing molten salt, electrolytic cell, and
process for producing ti using said method
Abstract
The present invention provides a method for electrolyzing molten
salt that can enhance the concentration of metal-fog forming metal
in the molten salt by carrying out the electrolysis under
conditions that the molten salt containing the chloride of
metal-fog forming metal is supplied from one end of an electrolytic
cell to a space between an anode and a cathode in a continuous or
intermittent manner to provide a flow rate in one direction to the
molten salt in the vicinity of the surface of the cathode and thus
to allow the molten salt to flow in one direction in the vicinity
of the surface of the cathode. According to the present invention,
while high current efficiency is maintained, only the molten salt
enriched with metal-fog forming metal such as Ca can be effectively
taken out. Further, this method can easily be carried out by using
the electrolytic cell according to the present invention.
Furthermore, the application of the method for electrolyzing molten
salt according to the present invention to the production of Ti by
Ca reduction can realize the production of metallic Ti with high
efficiency. Thus, the method for electrolyzing molten salt, the
electrolytic cell, and the process for producing Ti, each according
to the present invention, can be effectively utilized in the
production of Ti by Ca reduction.
Inventors: |
Ogasawara; Tadashi; (Hyogo,
JP) ; Yamaguchi; Makoto; (Hyogo, JP) ;
Uenishi; Toru; (Hyogo, JP) ; Hori; Masahiko;
(Hyogo, JP) ; Takemura; Kazuo; (Hyogo, JP)
; Dakeshita; Katsunori; (Hyogo, JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
37808663 |
Appl. No.: |
11/991072 |
Filed: |
August 22, 2006 |
PCT Filed: |
August 22, 2006 |
PCT NO: |
PCT/JP2006/316348 |
371 Date: |
February 27, 2008 |
Current U.S.
Class: |
205/359 ;
204/252; 205/398 |
Current CPC
Class: |
C25C 3/02 20130101; C22B
34/1268 20130101; C22B 34/129 20130101; C22B 5/04 20130101; C25C
7/005 20130101 |
Class at
Publication: |
205/359 ;
204/252; 205/398 |
International
Class: |
C25B 1/26 20060101
C25B001/26; C25B 9/00 20060101 C25B009/00; C25C 3/28 20060101
C25C003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2005 |
JP |
2005-248603 |
Claims
1-7. (canceled)
8. A method for electrolyzing molten salt employing an electrolytic
cell provided, between an anode and a cathode, with a diaphragm or
a partition wall configured so that part of the molten salt can
communicate therethrough, comprising: supplying molten salt
containing the chloride of a metal-fog forming metal to a space
between said cathode and said diaphragm or partition wall in a
continuous or intermittent manner; and, providing a flow rate in
one direction to the molten salt in the vicinity of the surface of
the cathode to thereby cause the formation of a flow of the molten
salt in one direction along the surface of the cathode, and
carrying out the electrolyzing along the flow to thereby enhance
the concentration of the metal-fog forming metal.
9. The method for electrolyzing a molten salt according to claim 8,
in which the method entails employing an electrolytic cell
comprised of an anode and a cathode where the surfaces of said
anode and cathode are disposed facing each other in a substantially
vertical direction.
10. The method for electrolyzing molten salt according to claim 8,
wherein the concentration of the metal-fog forming metal in the
molten salt in the electrolytic cell is controlled so that it may
be at a level lower than the saturation solubility.
11. An electrolytic cell, comprising: an electrolytic cell
container elongated in one direction and intended for retaining
molten salt containing the chloride of the metal-fog forming metal;
an anode and a cathode disposed along the lengthwise direction of
the electrolytic cell container; a diaphragm or a partition wall,
being configured so that part of the molten salt can communicate
therethrough, as disposed between said anode and said cathode; a
molten salt feeding port capable of supplying the molten salt to a
space between said cathode and said diaphragm or said partition
wall, as provided at one end of the electrolytic cell container;
and a molten salt drawing out port provided at the other end
thereof for drawing out the molten salt increased in Ca
concentration as formed by electrolysis of the molten salt.
12. The electrolytic cell according to claim 11, wherein the
surfaces of said anode and said cathode are disposed facing each
other in a substantially vertical direction.
13. A process for producing Ti, comprising: a reduction step of
causing TiCl.sub.4 to react with Ca in a molten salt containing
CaCl.sub.2 and further Ca dissolved therein to thereby cause
formation of Ti particles in said molten salt; a separation step of
separating said Ti particles formed in said molten salt from said
molten salt; and an electrolysis step of electrolyzing molten salt
decreased in Ca concentration in association with formation of Ti
particles to increase the Ca concentration, wherein the molten salt
increased in Ca concentration as formed in the electrolysis step is
used for reduction of TiCl.sub.4 in the reduction step, and wherein
the method for electrolyzing molten salt according to claim 8 is
applied in said electrolysis step.
14. The method for electrolyzing molten salt according to claim 9,
wherein the concentration of the metal-fog forming metal in the
molten salt in the electrolytic cell is controlled so that it may
be at a level lower than the saturation solubility.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for electrolyzing
molten salt by which molten salt increased in Ca concentration by
electrolyzing molten salt containing the chloride of metal-fog
forming metal (e.g. Ca, Li, Na, Al, etc.), in particular
CaCl.sub.2, can be obtained, an electrolytic cell for use in
carrying out that method, and a process for producing Ti using that
method.
BACKGROUND ART
[0002] A common industrial process for producing metallic Ti is the
Kroll process comprising reducing TiCl.sub.4 with Mg. In this Kroll
process, metallic Ti is produced via a reduction step and a vacuum
separation step. In the reduction step, liquid-form TiCl.sub.4 fed
from above in a reaction vessel is reduced by molten Mg, whereupon
granular metallic Ti is formed and then gradually moves downward
and settles to give metallic Ti in sponge form. In the vacuum
separation step, the unreacted Mg and the byproduct MgCl.sub.2 are
removed from the metallic Ti in sponge form inside the reaction
vessel.
[0003] In the production of metallic Ti by the Kroll process, it is
possible to produce high-purity products. However, since the
process is a batch-wise one, the production costs increase and the
prices of the products become very high. One of the causes for the
increased production costs is the difficulty in increasing the rate
of feeding of TiCl.sub.4.
[0004] While several reasons therefor are conceivable, one is that
when the rate of feeding of TiCl.sub.4 is excessively increased,
TiCl.sub.4 is supplied from the above onto the MgCl.sub.2 failing
to move downwards and remaining on the liquid surface and,
therefore, the TiCl.sub.4 fed is partly discharged from the
reaction vessel in the form of unreacted TiCl.sub.4 gas and/or
insufficiently reduced TiCl.sub.3 gas, among others; resulting in a
reduced efficiency in utilization of TiCl.sub.4.
[0005] Further, in the Kroll process, the reaction is carried out
only in the vicinity of the liquid surface of the molten Mg in the
reaction vessel, so that the heat-liberating area is narrow.
Therefore, cooling will not be able to keep up with the supply of
TiCl.sub.4 if fed at a high rate; this is also a major reason for
the rate of feeding of TiCl.sub.4 being limited.
[0006] Further, due to the wettability (stickiness) of molten Mg,
the formed Ti powder moves downwards in a flocculated state and
even during moving downwards, sintering and resulting grain growth
occur by the heat which the high-temperature molten liquid has,
thus rendering it difficult to discharge the same out of the
reaction vessel. As a result, the production of metallic Ti cannot
be carried out continuously, and the productivity is fettered.
[0007] As regards another process for producing Ti other than the
Kroll process, the specification of U.S. Pat. No. 2,205,854
describes that Ca can be used as a reducing agent other than Mg for
reduction of TiCl.sub.4. And, as a process for producing Ti using
the reduction reaction with Ca, a method is described in the
specification of U.S. Pat, No. 4,820,339 (hereinafter referred to
as "Document 1") which comprises retaining CaCl.sub.2 in molten
salt form in a reaction vessel, feeding a metallic Ca powder into
the molten salt from the above and allowing the Ca powder to
dissolve in the molten salt and, at the same time, supplying
TiCl.sub.4 gas from the below for causing the molten Ca to react
with TiCl.sub.4 in molten CaCl.sub.2 salt.
[0008] However, the process described in Document 1 cited above
cannot become effective as a commercial process for producing Ti
since the metallic Ca powder to be used as the reducing agent is
very. expensive and, if this is purchased and used, the production
costs will become higher as compared with the Kroll process. In
addition, it is very difficult to handle Ca which is highly
reactive; this is also an important factor hindering the industrial
use of the process for producing Ti by Ca reduction.
[0009] As a further process for producing Ti, the Olson's process
is described in the specification of U.S. Pat. No. 2,845,386
(hereinafter referred to as "Document 2") which comprises directly
reducing TiO.sub.2 with Ca without going through TiCl.sub.4
processing This process is a kind of direct oxide reduction method.
However, the use of high-purity TiO.sub.2, which is expensive, is
inevitable in this process.
[0010] On the other hand, the present inventors considered that the
reduction of TiCl.sub.4 with Ca should be essential for the
establishment of an industrial process for producing Ti by Ca
reduction and that it would be necessary to economically replenish
the Ca in the molten salt consumed in the reduction reaction, and
they proposed, in Japanese Patent Application Publication No.
2005-133195 (hereinafter referred to as "Document 3") and Japanese
Patent Application Publication No. 2005-133196 (hereinafter
referred to as "Document 4"), a process which utilizes the Ca
formed by electrolysis of molten CaCl.sub.2 and recycling this Ca,
namely "OYIK process". Document 3 cited above describes a process
in which Ca is formed and replenished by electrolysis and the
Ca-enriched molten CaCl.sub.2 is introduced into a reaction vessel
and used for the formation of Ti particles by Ca reduction, and
Document 4 cited above further discloses a method of effectively
inhibiting the back reaction resulting from electrolysis through
the use of an alloy electrode (e.g. Mg--Ca alloy electrode) as a
cathode.
DISCLOSURE OF INVENTION
[0011] As mentioned above, a number of research and development
works have so far been made concerning processes for producing Ti
other than the Kroll process. In particular, in the OYIK process
proposed by the present inventors, Ca in the molten salt is
consumed with the progress of the reduction reaction of TiCl.sub.4
but when that molten salt is electrolyzed, Ca is formed in the
molten salt; when the thus-obtained Ca is reused in the reduction
reaction, Ca replenishment from outside becomes unnecessary and,
furthermore, it is unnecessary to isolate and discharge Ca singly
so that the economical efficiency is enhanced.
[0012] Accordingly, the present inventors made investigations
concerning the step of electrolyzing molten CaCl.sub.2 as part of
the efforts to further develop a process for producing metallic Ti,
wherein the core concept thereof is based on the OYIK process and
the operation can be carried out more efficiently and reliably. The
process for producing Ti or Ti alloys according to the present
invention is named "OYIK-II process" after the initials of the four
persons "Ogasawara, Yamaguchi, Ichihashi and Kanazawa" who had been
deeply involved in coming up with an idea, development and
completion of that process.
[0013] It is an object of the present invention to provide: a
method for electrolyzing molten salt which makes it possible to
carry out the recovery of a highly concentrated Ca-containing
molten salt in obtaining the molten salt increased in Ca
concentration by electrolyzing the molten salt containing chloride
of a metal fog forming metal such as Ca, Li, Na or Al, in
particular CaCl.sub.2, and by which high current efficiency can be
maintained and a large amount of molten CaCl.sub.2 can be
electrolyzed in a continuous manner; an electrolytic cell for use
in carrying out such method; and a process for producing Ti by
applying that method.
[0014] To accomplish the above object, the present inventors made
detailed investigations concerning the shape of the electrolytic
cell container, the shape of the electrode, the electrolysis
conditions and the distance between electrodes, among others, using
molten CaCl.sub.2 and, as a result, have now completed the present
invention.
[0015] The gist of the present invention consists in (1) a method
for electrolyzing molten salt, (2) an electrolytic cell, and (3) a
process for producing Ti using that method, as defined below.
(1) A method for electrolyzing a molten salt by which the
electrolysis is carried out in such a state that molten salt
containing the chloride of metal-fog forming metal is supplied from
one end of an electrolytic cell to a space between an anode and a
cathode in a continuous or intermittent manner to provide a flow
rate in one direction to the molten salt in the vicinity of the
surface of the cathode and thus to allow the molten salt to be
electrolyzed in the vicinity of the surface of the cathode, while
allowing flowing in one direction therein, to thereby enhance the
metal-fog forming metal concentration.
[0016] The "metal-fog forming metal" so referred to herein is the
metal capable of being itself dissolved in metal chloride, such as
Ca, Li, Na or Al (namely, Ca is soluble in CaCl.sub.2 or Li is
soluble in LiCl.sub.2), and capable of reducing TiCl.sub.4.
[0017] When, in applying the method for electrolyzing molten salt,
an electrolytic cell, in which the surfaces of the anode and
cathode are disposed facing each other in a substantially vertical
direction and a diaphragm or a partition wall configured to allow
part of the molten salt to communicate therethrough is provided
between the anode and cathode, is used, the recovery of chlorine
gas generated on the anode side becomes facilitated. Furthermore,
the back reaction which is the reaction between the metal-fog
forming metal (e.g. Ca) formed by electrolysis and chlorine (Cl) to
again form CaCl.sub.2 can be prevented; hence the use of such
electrolytic cell is preferable (hereinafter referred to as "a
first mode of embodiment").
[0018] When an embodiment mode is employed such that: the cathode
is hollow and has gaps or holes; the molten salt can flow
therethrough from the surface of the cathode into the inside
thereof; and the molten salt enriched with metal-fog forming metal,
which flows into the inside of the cathode, can be drawn out of the
electrolytic cell, it becomes possible to effectively inhibit the
back reaction (hereinafter referred to as "a second mode of
embodiment").
[0019] When the metal-fog forming metal concentration in the molten
salt in the electrolytic cell is controlled so that it may be at a
level lower than the saturation solubility, it becomes possible to
increase the Ca concentration and increase the rate of formation of
Ti and, further, prevent clogging in the inside of the electrolytic
cell and like troubles (hereinafter referred to as "a third mode of
embodiment").
(2) An electrolytic cell which comprises an electrolytic cell
container elongated in one direction and intended for retaining the
molten salt containing the chloride of metal-fog forming metal, and
an anode and a cathode disposed along the lengthwise direction of
the electrolytic cell container, and is provided with a molten salt
feeding port at one end of the lengthwise direction of the
electrolytic cell container for supplying the molten salt to a
space between the anode and cathode and with a molten salt drawing
out port at the other end thereof for drawing out the molten salt
increased in Ca concentration as formed by electrolysis of the
molten salt.
[0020] When the electrolytic cell is configured such that the
surfaces of the anode and cathode are disposed facing each other in
a substantially vertical direction and, further, a diaphragm or a
partition wall allowing part of the molten salt to communicate
therethrough is disposed between the anode and cathode, it can
suitably be used in applying the method of electrolysis according
to the above-mentioned first mode of embodiment.
(3) A process for producing Ti which comprises: a reduction step of
causing TiCl.sub.4 to react with Ca in molten salt containing
CaCl.sub.2 and further Ca dissolved therein to thereby cause
formation of Ti particles in the molten salt; a separation step of
separating the Ti particles formed in the molten salt therefrom;
and an electrolysis step of electrolyzing the molten salt decreased
in Ca concentration in association with formation of Ti particles
to thereby increase the Ca concentration, wherein the molten salt
increased in Ca concentration as formed in the electrolysis step is
used for reduction of TiCl.sub.4 in the reduction step, and wherein
the method for electrolyzing molten salt as defined above in (1) is
applied in the electrolysis step.
[0021] The method for electrolyzing molten salt according to the
present invention is the one comprising electrolyzing molten salt
while the molten salt is caused to flow in one direction in the
vicinity of the surface of the cathode and recovering the molten
salt enhanced in metal-fog forming metal concentration on the
outlet side of the electrolytic cell. This electrolysis method
makes it possible to suppress the back reaction and maintain high
current efficiency and, at the same time, effectively take out only
the molten salt enriched in such a metal-fog forming metal as Ca
and, further, continuously electrolyze a large amount of molten
CaCl.sub.2. This method can be applied with ease using the
electrolytic cell according to the present invention.
[0022] Further, when the method for electrolyzing molten salt
according to the present invention is applied to the production of
Ti by Ca reduction, a molten salt enriched in Ca can be obtained in
a relatively stable manner, so that metallic Ti can be produced
efficiently.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a vertical sectional view illustrating a
constitutional example of the principal parts of an electrolytic
cell according to the present invention.
[0024] FIG. 2 is a partial schematic representation of another
constitutional example of an electrolytic cell according to the
present invention in which a hollow cathode is used.
[0025] FIG. 3 is a diagram showing, by way of example, the steps in
applying the process for producing Ti according to the present
invention.
BEST MODES FOR CARRYING OUT THE INVENTION
[0026] In the following, the method for electrolyzing molten salt,
the electrolytic cell and the process for producing Ti using that
method, each according to the present invention, are described more
specifically, with reference to the drawings. Since, when forming
Ti by reduction of TiCl.sub.4 using the method for electrolyzing
molten salt according to the present invention, all metal-fog
forming metals act and behave in the same manner, the case where
the metal fog forming metal is Ca is described in the
following.
[0027] FIG. 1 is a vertical sectional view illustrating a
constitutional example of the principal parts of an electrolytic
cell to be used in applying the method for electrolyzing molten
salt according to the present invention.
[0028] This electrolytic cell 1 comprises: an electrolytic cell
container 1a having a tubular (cylindrical) form elongated in one
direction for retaining CaCl.sub.2-containing molten salt; a
similarly cylindrical anode 2 and a round column-like cathode 3
disposed in the container 1a along the lengthwise direction of the
electrolytic cell container 1a; a molten salt feeding port 6 at one
end (bottom 4) of the lengthwise direction of the electrolytic cell
container 1a; and a molten salt drawing out port 7 at the other end
(upper wall 5) thereof. The surfaces of the anode and cathode are
disposed facing each other in a substantially vertical direction
and, further, a diaphragm 8 for preventing the passage of Ca formed
by electrolysis of the molten salt is provided between the anode 2
and cathode 3. Furthermore, the outer surface of the anode 2 is
provided with a cooling device 9.
[0029] The method for electrolyzing molten salt according to the
present invention is characterized in that the molten salt
containing the chloride (CaCl.sub.2) of a metal-fog forming metal
(Ca) is supplied from one end of the electrolytic cell to a space
between the anode and cathode in a continuous or intermittent
manner to thereby provide a flow rate in one direction to the
molten salt in the vicinity of the surface of the cathode and the
Ca concentration in the molten salt is increased by carrying out
the electrolysis while the molten salt is caused to flow in one
direction in the vicinity of the surface of the cathode.
[0030] Thus, according to the method for electrolyzing molten salt
according to the present invention, CaCl.sub.2-containing molten
salt is first supplied from one end of the electrolytic cell 1 to a
space between the anode 2 and cathode 3 in a continuous or
intermittent manner. The "CaCl.sub.2-containing molten salt" so
referred to herein indicates molten CaCl.sub.2 alone or molten salt
with additive, to molten CaCl.sub.2, such as KCl, CaF.sub.2 or the
like for lowering the melting point and adjusting the viscosity and
so forth. Hereinafter, such molten salt is referred to simply as
"molten salt".
[0031] Since the electrolytic cell 1 has a shape elongated in one
direction (in the example shown, a vertically elongated tubular
(cylindrical) shape), it is possible to provide a flow rate in one
direction to the molten salt in the vicinity of the surface of the
cathode 3 and thereby cause the molten salt to flow in one
direction in the vicinity of the surface of the cathode 3 by
supplying the molten salt from one end of the electrolytic cell 1
to a space between the anode 2 and cathode 3 in a continuous or
intermittent manner. In this case, it is enough to attain such a
condition that at least a partial molten salt staying in the
vicinity of the surface of the cathode 3 flows in one direction, or
the whole molten salt between the anode 2 and cathode 3 may bodily
flow in one direction. The phrase "in the vicinity of the surface
of the cathode" refers to the region adjacent to the surface of the
cathode where Ca formed on the surface of the cathode exists.
[0032] Although the molten salt is generally supplied continuously,
the molten salt may be fed intermittently in relation to the
subsequent step, for instance; namely, the supply of the molten
salt may be temporarily suspended and then resumed again. When the
molten salt supply is temporarily suspended, the flow of the molten
salt in the vicinity of the surface of the cathode is also
suspended. Therefore, the "flow rate" on the occasion of "providing
a flow rate in one direction to the molten salt in the vicinity of
the surface of the cathode"in the strict sense of the term includes
also the no-flow condition in which the flow rate is 0 (zero).
[0033] The molten salt is then electrolyzed. That is, the molten
salt is electrolyzed to form Ca on the surface of the cathode while
it is caused to flow in one direction in the vicinity of the
surface of the cathode. Since the electrolytic cell 1 has a shape
elongated in one direction and, in the example shown in FIG. 1, the
distance between the anode 2 and cathode 3 is relatively short to
suppress the electrolytic voltage, the Ca-enriched molten salt
alone can be drawn out effectively while the molten salt in
proximity to the molten salt feeding port 6, which is low in Ca
concentration, is prevented from mixing with the molten salt in
proximity to the molten salt drawing out port 7, which has an
increased Ca concentration as a result of electrolysis.
[0034] The technology described in Document 2 cited above uses Ca
as the reducing agent but is a direct reduction method for reducing
TiO.sub.2, not TiCl.sub.4, with Ca to Ti and thus is different from
the method of electrolysis according to the present invention.
Furthermore, in the direct reduction method described in Document 2
cited above, the carbon electrode to be used as the anode is
consumed as CO.sub.2 and, in addition, titanium carbide (TiC) is
formed in the molten salt, so that the resulting Ti is adulterated
with C-contaminated Ti and the workability is deteriorated;
problems may be encountered in using such Ti for making wrought
products.
[0035] Further, Document 2 cited above describes a technology of
"forming a flow of molten salt in the vicinity of the cathode in
forming Ti by Ca reduction in molten salt". However, there is no
description suggesting the technological philosophy of the present
invention that: the anode and cathode should be disposed facing
each other along the lengthwise direction of the electrolytic cell;
the molten salt should be caused to flow in one direction in the
vicinity of the surface of the cathode or, where a diaphragm or the
like is provided, in the cathode compartment formed between the
surface of the cathode and the diaphragm; and the electrolysis
should be carried out under such conditions to thereby recover the
molten salt increased in Ca concentration on the outlet side of the
electrolytic cell.
[0036] Therefore, the method for electrolyzing molten salt
according to the present invention and the technology described in
Document 2 are quite different from each other even if they are
common in that the molten salt is caused to form a unidirectional
flow in the electrolytic cell.
[0037] In the first mode of embodiment thereof, the method for
electrolyzing molten salt according to the present invention is the
one using an electrolytic cell in which the surfaces of the anode
and cathode are disposed facing each other in a substantially
vertical direction and a diaphragm or a partition wall configured
so that part of the molten salt can communicate therethrough is
provided between the anode and cathode. The term "substantially" in
the above-mentioned phrase "in a substantially vertical direction"
means "almost" or "approximately", and the "substantially vertical
direction" indicates the vertical direction or a direction slightly
slanted from the vertical direction.
[0038] The electrolysis method in the first mode of embodiment can
be carried out more preferably by using such an electrolytic cell
as shown by way of example in FIG. 1. While the system employed in
the electrolytic cell shown in FIG. 1 comprises feeding CaCl.sub.2
from the lower side of the electrolytic cell 1 and drawing out the
same from the upper side thereof, it is also possible to employ the
system comprising supplying CaCl.sub.2 from the upper side of the
electrolytic cell 1 and drawing out the same from the lower side
thereof.
[0039] In the electrolytic cell to be used in this electrolysis
method, the surfaces of the anode and cathode are disposed facing
each other in a substantially vertical direction and, on the other
hand, the molten salt in the vicinity of the surface of the cathode
is provided with a flow rate in one direction and the direction of
the flow of the molten salt is vertical, hence chlorine gas
generated on the anode side easily moves upwards to the surface and
can be recovered with ease.
[0040] Usable as the diaphragm to be disposed between the anode and
cathode is, for example, a porous ceramic body comprising yttria
(Y.sub.2O.sub.3). A porous ceramic body prepared by firing yttria
has selective permeability such that ions such as Ca and chlorine
can permeate therethrough but metallic Ca cannot and, further, has
good resistance to calcium reduction such that it cannot be reduced
even with Ca strong in reducing power Therefore, the porous ceramic
body is suited for use as the diaphragm in applying the method for
electrolyzing molten salt according to the present invention.
[0041] When an electrolytic cell provided with such a diaphragm
between the anode and cathode is used, the back reaction, namely
the immediate reaction between the Ca formed on the cathode side
and the chlorine formed on the anode (graphite) side to again form
CaCl.sub.2, hardly occurs and the electrolysis can be carried out
with high current efficiency.
[0042] A partition wall configured so that part of the molten salt
can communicate therethrough may be used instead of the diaphragm.
The partition wall does not allow the passage of not only metallic
Ca but also such molten salt constituents as Ca and chlorine ions
but, when the partition wall is partially provided with slits or
holes through which the molten salt can communicate, it enables the
electrolysis and, on the other hand, restricts the passage of
metallic Ca to a certain extent, making it possible to suppress the
back reaction.
[0043] The second mode of embodiment of the method for
electrolyzing molten salt (including the first mode of embodiment)
according to the present invention is the one by which the cathode
is hollow and has gaps or holes through which the molten salt can.
flow from the surface of the cathode into the inside thereof (i.e.
inner hollow space) so that the Ca-enriched molten salt that flew
into the inside of the cathode can be drawn out of the electrolytic
cell.
[0044] FIG. 2 is a partial schematic representation of another
constitutional example of an electrolytic cell in which a hollow
cathode is used. As shown in FIG. 2, in this electrolytic cell 1,
an anode 2 and a hollow cathode 3a are disposed facing each other
in a substantially vertical direction along the lengthwise
direction of the electrolytic cell 1 and a diaphragm 8 is provided
between the anode 2 and cathode 3a. The cathode 3a is provided with
gaps or holes (not shown) through which the molten salt can flow
from the surface of the cathode into the inside thereof.
[0045] When a thus-constituted electrolytic cell is used and the
molten salt is drawn out from the upper side of the hollow section
of the cathode 3a, a flow of the molten salt from the outer surface
of the cathode to the inside thereof (inner hollow space) is
formed, as shown by outlined arrows, and the Ca formed on the outer
surface of the cathode 3a is directly taken into the inside of the
cathode 3a without dispersion or migration to the anode side. As a
result, the back reaction can be effectively prevented. The
electrolytic cell shown by way of example in FIG. 2 comprises a
diaphragm 8, so that the back reaction preventing effect is much
stronger as compared with the case where there is no diaphragm.
[0046] The size and positions, among others, of the gaps or holes
to be provided in the hollow cathode are not particularly limited.
They may properly be selected so that an effective molten salt flow
toward the inner surface of the cathode may be formed, taking into
consideration the distance between the surface of the anode (the
diaphragm surface when a diaphragm is provided) and the outer
surface of the cathode, the amount of the molten salt drawn out
(amount of the molten salt supplied) and other factors.
[0047] The third mode of embodiment of the method for electrolyzing
molten salt (including the first and second modes of embodiment)
according to the present invention is the one of electrolysis by
which the Ca concentration in the molten salt in the electrolytic
cell is controlled so that it may be at a level lower than the
saturation solubility. By saying "the Ca concentration is
controlled so that it may be at a level lower than the saturation
solubility" above, it is meant that the electrolysis is carried out
"under conditions such that the Ca concentration should be close to
the saturation solubility but should not be so much to allow Ca to
precipitate out".
[0048] More specifically, optimum electrolysis conditions, an
amount of molten salt to be drawn out per unit time and other
factors are determined empirically according to the shape of the
electrolytic cell container, the shapes of the electrodes, the
distance between poles and the like, so that "the conditions that
the Ca concentration should be close to the saturation solubility
but should not be so much to allow Ca to precipitate out" may be
satisfied at the site showing the maximum Ca concentration in the
electrolytic cell. In particular, when a diaphragm or partition
wall is used between the anode and cathode, the Ca concentration
becomes maximum in proximity to the molten salt drawing out port on
the cathode side. Therefore, by controlling the Ca concentration at
such site at a level lower than the saturation solubility, the
electrolytic operation may be carried out without allowing metallic
Ca to precipitate out at any site in the electrolytic cell.
[0049] When such a electrolysis method is employed, it is possible
to obtain the molten salt enriched in Ca to a level close to the
saturation solubility in a relatively stable manner while such
troubles as clogging in the inside of the electrolytic cell are
prevented.
[0050] In an exemplary mode of embodiment of the Ca enrichment
according to the present invention, when the temperature of
CaCl.sub.2 entering the electrolytic cell may be set at 800.degree.
C., the metallic Ca concentration in the molten CaCl.sub.2 can be
increased from 0% to a metallic Ca concentration of 1% in the
molten CaCl.sub.2 leaving the electrolytic cell. It is preferable
that the metallic Ca concentration (concentration A) in the molten
CaCl.sub.2 entering the electrolytic cell is from 0% to less than
1% and the metallic Ca concentration (concentration B) in the
molten CaCl.sub.2 leaving the electrolytic cell be not less than
0.1%. Considering the efficient utilization of Ca in the subsequent
step, the increment (concentration B-A) in metallic Ca
concentration in the electrolytic cell is preferably not less than
0.1% and not more than 5.0% (concentration including supersaturated
Ca), particularly preferably not less than 1.0%.
[0051] In carrying out the method for electrolyzing molten salt
according to the present invention, the heat of reaction is
generated in large quantities in the electrolytic cell and
therefore it is preferable that the heat is removed effectively.
More specifically, either in cases where the hollow cathode
mentioned above is used or in cases where such is not used, it is
preferable that a cooling device is disposed in the central part of
the cathode to remove the heat of reaction from the inside of the
cathode. A tubular heat exchanger, for instance, is suited for use
as the cooling device.
[0052] When a cooling device (heat exchanger) is disposed on the
anode side, the heat removal efficiency is further enhanced. The
cooling device 9 disposed so as to surround the anode 2 as shown in
FIG. 1 is an example.
[0053] For increasing the yield of Ca by increasing the current
supply in electrolysis, it is necessary to enlarge the surface area
for current supply. It is preferable that the inside surface of the
anode 2, namely the surface opposing the surface of the cathode in
the electrolytic cell 1 shown by way of example in FIG. 1, is
provided with minute concavo-convex irregularities to secure a
large surface area for current supply. Applicable as the means
therefor is, for example, grooving for forming grooves on the
electrode surface.
[0054] In accordance with the method for electrolyzing molten salt
according to the present invention, since the electrolysis is
carried out while the molten salt is caused to flow in one
direction in the vicinity of the surface of the cathode, a large
amount of the molten salt can be treated continuously.
[0055] The electrolytic cell according to the present invention is
an electrolytic cell to be used in carrying out the above-mentioned
method for electrolyzing a molten salt and is characterized in that
it comprises: an electrolytic cell container elongated in one
direction and intended for retaining a molten salt containing
CaCl.sub.2; an anode and a cathode disposed along the lengthwise
direction of the electrolytic cell container; a molten salt feeding
port at one end of the lengthwise direction of the electrolytic
cell container for supplying the molten salt to a space between the
anode and cathode; and a molten salt drawing out port at the other
end thereof for drawing out the molten salt increased in Ca
concentration as formed by electrolysis of the molten salt.
[0056] The electrolytic cell, shown by way of example in FIG. 1, is
one mode of embodiment of the electrolytic cell according to the
present invention, where the surfaces of the anode and cathode are
disposed facing each other in a substantially vertical direction
and a diaphragm is disposed between the anode and cathode. A
partition wall configured so that part of the molten salt can
communicate therethrough may be disposed therein instead of the
diaphragm. When the electrolytic cell shown in FIG. 1 is used, the
method for electrolyzing molten salt according to the present
invention can be suitably applied, as already mentioned
hereinabove.
[0057] The process for producing Ti according to the present
invention characterized by comprising: a reduction step of causing
TiCl.sub.4 to react with Ca in a molten salt containing CaCl.sub.2
and further Ca dissolved therein to thereby cause formation of Ti
particles in the molten salt; a separation step of separating the
Ti particles formed in the molten salt therefrom; and an
electrolysis step of electrolyzing the molten salt decreased in Ca
concentration in association with formation of Ti particles to
increase the Ca concentration, wherein the molten salt increased in
Ca concentration as formed in the electrolysis step is used for the
reduction of TiCl.sub.4 in the reduction step, and wherein the
method for electrolyzing the molten salt according to the present
invention is applied in the above-mentioned electrolysis step.
[0058] FIG. 3 is a diagram showing, by way of example, the steps in
applying the process for producing Ti according to the present
invention. As shown in FIG. 3, this process for producing Ti
comprises: the reduction step 10 of causing TiCl.sub.4 to react
with Ca in a molten salt containing CaCl.sub.2 and further Ca
dissolved therein to form Ti particles in the molten salt; the
separation step 11 of separating the Ti particles formed in the
molten salt from the molten salt; and the electrolysis step of
electrolyzing the molten salt reduced in Ca concentration in
association with formation of the Ti particles to increase the Ca
concentration. In the process for producing Ti according to the
present invention, the above-mentioned method for electrolyzing the
molten salt is applied in this electrolysis step and, therefore, an
electrolytic cell 1 for use in this electrolysis step is included
therein.
[0059] The electrolytic cell 1 used here comprises: an electrolytic
cell container 1a having a vertically elongated cylindrical shape;
an anode 2 and a cathode 3 disposed along the lengthwise direction
of the electrolytic cell container 1a; and a diaphragm 8 disposed
between the anode 2 and cathode 3. The electrolytic cell 1 is
provided, at the upper end thereof, with a molten salt feeding port
(not shown) for supplying molten salt to a space between the anode
2 and cathode 3 and, at the lower end thereof, with a molten salt
drawing out port (not shown) for drawing out the molten salt
increased in Ca concentration as formed by electrolysis of the
molten salt.
[0060] The CaCl.sub.2-containing molten salt supplied from the
upper end of the electrolytic cell 1 moves downward within the
electrolytic cell and electrolyzed during movement, whereby Ca is
formed. The Ca concentration in the molten salt is increased as the
molten salt moves downward. During electrolysis, the back reaction
is suppressed by the diaphragm 8 disposed between the anode 2 and
cathode 3 and, thus, the current efficiency is maintained at a high
level. Further, during operation, the Ca concentration in the
molten salt is controlled at a level lower than the saturation
solubility, namely so that the Ca concentration may be close to the
saturation solubility but not so much to allow Ca to precipitate
out. In addition, since the electrolytic cell 1 is a vertical type,
the chlorine gas generated on the anode side can be recovered with
ease.
[0061] The thus-obtained molten salt enriched in Ca is drawn out
through the molten salt drawing out port at the lower end of the
electrolytic cell 1 and transferred to the reduction step 10. In
the reduction step 10, TiCl.sub.4 gas is caused to react with Ca in
the molten salt enriched in Ca, whereby granular metallic Ti is
formed in the molten salt. As the reduction reaction proceeds in
the molten salt, the Ca in the molten salt is consumed while Ti is
formed and at the same time CaCl.sub.2 is formed as a
byproduct.
[0062] The Ti particles formed in the reduction step 10 are
transferred, together with the molten salt, to the separation step
11, and the Ti particles are separated from the molten salt.
Applicable to the separation are a solid-liquid separation
procedure using such as a high-speed decanter (continuous
centrifugation) system, a thickener system or the like. If, though
not shown, the reaction vessel to be used in this reduction step 10
is constituted so that the byproduct CaCl.sub.2-containing molten
salt may be discharged out of the vessel, it is also possible to
transfer the molten salt discharged from this reduction step 10
directly to the electrolysis step (cf. Documents 3 and 4 cited
above).
[0063] While the Ti powder obtained in the Kroll process is in an
agglomerated state, the Ti particles obtained in the reduction step
10 is hardly agglomerated and is hardly adhering to the vessel, so
that they are easily taken out of the vessel; the recovered Ti
particles, as such, can be transferred to the melting step in which
they are heated and melted to provide a Ti ingot 12.
[0064] On the other hand, the remaining molten salt reduced in Ca
concentration after separation and recovery of the Ti particles is
sent to the electrolysis step, in which it is subjected to
electrolysis treatment in the above-mentioned electrolytic cell 1
and the resulting molten salt enriched in Ca is again used for
reducing TiCl.sub.4 in the reduction step 10.
[0065] In the process for producing Ti according to the present
invention, the molten salt enriched in Ca to a level close to the
saturation solubility is obtained in the electrolysis step in a
relatively stable manner, so that metallic Ti can be produced with
good efficiency; and, further, Ca formed by continuous electrolysis
of a large amount of molten salt can be supplied to the reduction
step. Therefore, the process can be also suited for mass
production.
INDUSTRIAL APPLICABILITY
[0066] The method for electrolyzing molten salt according to the
present invention is the one for carrying out electrolysis while
the molten salt is caused to flow in one direction in the vicinity
of the surface of the cathode and, according to this method of
electrolysis, high current efficiency can be maintained and only
the molten salt enriched in such metal-fog forming metal as Ca can
be taken out effectively. This electrolysis method can be carried
out with ease using the electrolytic cell according to the present
invention. Further, when the method for electrolyzing molten salt
according to the present invention is applied to the production of
Ti by Ca reduction, a Ca-enriched molten salt is obtained in a
relatively stable manner and metallic Ti can be produced with good
efficiency. Therefore, the method for electrolyzing molten salt,
the electrolytic cell, and the process for producing Ti in which
said electrolysis method is applied, each according to the present
invention, can be effectively utilized in the production of Ti by
Ca reduction.
* * * * *